1. Field of the Invention
This invention relates to a fuel control system for integral rocket-ramjets which is automatically actuated upon transition from rocket to ramjet propulsion and which provides the limiting functions necessary to prevent the engine, air inlet and vehicle from operating in unacceptable regions. The fuel control modulates thrust by controlling fuel flow to perform the functions of ramjet light-off, inlet margin limiting, maximum fuel-to-air ratio limiting, vehicle velocity or Mach number limiting, and lean blowout limiting. Inlet margin limiting and vehicle velocity or Mach number limiting are performed in a closed loop fashion to provide extremely accurate control, particularly during the period immediately after ramjet light-off.
2. Description of the Prior Art
The concept of ramjet propulsion for vehicles was initiated in the early 1900's and practical development evolved in the 1940's. Since then numerous advances have been made in this technology, and applications to advanced missiles is predicted for the future.
Rockets have been known for many centuries and led to practical applications during World War II and thereafter, with both solid fueled and liquid fueled rockets used as weapons and for space exploration.
The marriage of the ramjet and rocket took place during the 1960's when the integral rocket-ramjet (IRR) was developed for missile applications. In 1967, the low altitude short-range missle, LASRM, was developed. The basic IRR is a combined propulsion system. Ramjet fuel is sealed off from the rocket fuel so that the IRR starts out as a pure rocket engine using rocket fuel in the ramjet combustion chamber, and a rocket nozzle inside the ramjet nozzle. During the rocket boost, the ramjet air inlets are typically covered with blow-off fairings and the air openings to the combustion chamber are sealed off with blow-off plugs. When the rocket fuel burns out, the blow-off fairings, inlet plugs and rocket nozzle are ejected leaving a ramjet propulsion system which is then ignited. Numerous configurations of IRR's have been developed, and this invention is applicable to any IRR in which a liquid fuel is fed to the ramjet combustion chamber and ignited upon termination of the rocket phase.
The basic ramjet consists of a supersonic air inlet, a combustor, a fuel supply system and an exhaust nozzle. The supersonic air inlet admits air to the engine, reduces the air velocity, and interfaces with the combustor which develops combustor pressure. The combustor adds heat and mass to the air by burning the fuel from the fuel supply and thereby increases combustor pressure. The nozzle converts combustion chamber pressure to kinetic energy to produce thrust.
The fuel to the ramjet is supplied from a storage tank by pumping or pressurization. A fuel control modulates fuel flow to prevent the engine, inlet and vehicle from operating in unacceptable regions. The control must permit thrust modulation over as large a range as is practical without exceeding the operating constraints to optimize vehicle performance. The fuel control matches fuel flow with airflow to maintain the fuel-to-air ratio within limiting values for both lean and rich mixtures. Operation is closely interrelated with conditions in both inlet and combustor. The fuel control must also maintain an appropriate initial flow of fuel during transition from rocket to ramjet operation, control inlet pressure margin, and limit flight Mach number. A ramjet fuel control may be considered an air inlet control in that it positions the shock wave at a desirable location in the ramjet inlet and meters fuel as required to maintain that shock position for the inlet margin limiting region of the flight envelope.
Numerous fuel controls for ramjets are known in the prior art, most of which have disadvantages such as inability to provide proper fuel flow over the large range of operating conditions encountered during high performance ramjet operation. More specifically, prior art ramjet fuel controls did not take into account the effect of the shock wave produced at the air inlet and thus often encountered operating conditions where vehicle performance deteriorated.
The present invention improves high performance ramjet operation by scheduling fuel via a novel fuel control system in which some functions are performed open loop, while the more critical functions are performed in a more accurate, closed loop fashion. The fuel schedules prevent the engine, inlet and vehicle from operating in inefficient or unacceptable regions, and provide thrust modulation over a wide range of operating conditions. The control is adaptable to a wide variety of vehicle configurations, and has the advantages of low cost, high reliability, and low weight and volume.
The basic control parameters are derived using electronic devices, and are adapted to digital implementation, while the actual fuel metering is performed by proven hydromechanical controls.
It is thefore an object of this invention to provide a ramjet fuel control which meters fuel to the ramjet combustor to insure safe ram burner light-off and transition from rocket to ramjet propulsion.
Another object of this invention is a ramjet fuel control which meters fuel to the ramjet combustor via a closed loop control to prevent the air inlet from operating in an unstable region.
A further object of this invention is a ramjet fuel control which provides both maximum and minimum fuel-to-air ratio limits for optimizing operation and preventing lean burner blow-out.
Another object of this invention is a ramjet fuel control which provides a closed loop maximum vehicle Mach number control.
A still further object of this invention is a ramjet fuel control in which closed loop Mach number and air inlet controls modulate fuel flow via a control parameter-defined as the ratio of combustion pressure to a reference pressure.
Another object of this invention is a ramjet fuel control in which scheduled values of the ratio of fuel flow to combustion chamber pressure are used to provide the desired fuel flow.